Send the details (other account's username and more information if you have it) to one of the forum administrators (JadeHellbringer and God and Davion) for them to sort out what they want to do, or just send them to me and I'll pass the information along. Don't post it in this thread, please.
In general, if you lose access to your account, the best thing to do is find a way to get in touch with a moderator or administrator. Even pinging the BattleTech social media accounts for CGL will probably work - the guys managing them can forward something over to us. The sysadmins can do things to help you recover your account, like change the email address or resetting the password for you.